Modern manufacturing quality control facilities use machines called Coordinate Measuring Machines (a.k.a. CMMs). CMMs are used to accurately measure 3-dimensional objects by lightly touching designated surfaces with one or more styli. Each time a CMM stylus touches the object, a computer attached to the CMM instantaneously records the position of the stylus to a high degree of precision. By collecting enough of these positions (“points”), an engineer can quickly and accurately determine the object's conformance to specifications.
CMMs are computer-controlled. A CMM programmer enters a program into the computer that tells the machine where to touch the object. In the case of large and/or complex objects (for example, an automobile engine), CMM programs involve thousands of touches by dozens of different size styli. Often, several individual styli are grouped together as “clusters”.
About 20 years ago, CMM manufacturers developed automatic stylus changing devices. These allowed a CMM to run uninterrupted during a lengthy program, automatically switching out styli as needed to measure all the features of a large object. Previously, the machine needed to be stopped and new styli loaded by hand. These stylus changing devices on CMMs are analogous to cutting tool changers on CNC (Computer Numerical Control) machine tools.
A significant difference between stylus changers and cutting tool changers is that stylus changers must be repeatable with regards to styli orientation relative to the CMM axes. Cutting tools, being round, are not affected by their radial orientation to the machine axes. CMM styli clusters, on the other hand, do not rotate. They must always be positioned in exactly the same way in the machine, or measurement results are useless.
To repeatably and accurately orient the stylus cluster each time, CMM manufacturers developed highly repeatable “adapter plates” to hold stylus clusters. These adapter plates employ a threaded cube from which individual styli protrude in various directions. This threaded cube can be rotated axially with regards to the adapter plate, allowing the styli access to points at any angle. The adapter plate is fixed to the CMM by means of an electromagnet. By de-energizing the electromagnet, a plate is released into a rack. The machine then moves to another plate, re-energizes the electromagnet, which causes the plate to lock into place in the machine.
CMM operators often need the threaded cube on the adapter plate to be oriented exactly square to the CMMs axes of motion. By doing so, all styli will, in turn, be aligned to the machine's X-Y-Z axes. This greatly simplifies machine set-up and programming.
However, what is needed is an improved coordinate measuring apparatus which may be more precise and efficient to use than the prior art coordinate measuring devices. Additionally, a need exists for a more compact and simple coordinate measuring apparatus.